WASHINGTON (MarketWatch) — The day after she claimed the Democratic nomination, Hillary Clinton told an interviewer that she was not going to trade insults with Donald Trump but just keep telling people how “he’s unqualified to be president” and “temperamentally unfit to be commander-in-chief.”

Even MSNBC anchor Chris Matthews, an ardent Clinton fan, had to chuckle at the oxymoronic quality of this statement.

Clinton, who has refused to face the press in a general news conference for months because she has no answers to questions about her private email server and Clinton Foundation donations, has apparently decided that attacking Trump’s temperament is the most convincing case she can make for voters to pick her.

But the temperament issue could backfire as a new book by a former Secret Service agent assigned to the White House detail during Bill Clinton’s administration charges that the former first lady is “erratic, uncontrollable and occasionally violent” and “lacks the integrity and temperament” to serve as president.

Pre-sell orders for Gary Byrne’s, “Crisis of Character,” set for release June 28, have already rocketed the book to the top of Amazon’s best-seller list, where it vies with J.K. Rowling’s new Harry Potter book for first place.

Byrne says he was “sickened” by what he saw in the Clinton White House and is convinced that Hillary Clinton should not be president.

“From the bottom of my soul I know this to be true,” Byrne wrote in excerpts obtained by the New York Post. “And with Hillary’s latest rise, I realize that her own leadership style — volcanic, impulsive, enabled by sycophants, and disdainful of the rules set for everyone else — hasn’t changed a bit.”

Harsh words that Clinton spokesman Nick Merrill quickly dismissed, saying the book “should be put in the fantasy section of the book store.”

And yet the new book is just the latest in a series of leaks and stories over the years about Hillary Clinton’s temper — including anecdotes of her hurling a vase at her husband and cursing and demeaning Secret Service agents.

If you like, you can simply impeach the credibility of Byrne, even though he has a 30-year career in federal law enforcement with stints at the U.S. Air Force Security Police and as a federal air marshal in addition to his Secret Service time.

His book might be more convincing to readers than the Clinton hatchet jobs by journalists like Ed Klein and Ronald Kessler, since it claims to be based on his own experience rather than hearsay from anonymous sources.

The timing, in any case, is propitious, as the presidential campaign moves into the general election phase. True or not, these stories gain wide credence among those looking for further reasons to dislike the Clintons — and these people are voters.

With her own unfavorability rating well over 50%, it might seem to be a risky strategy for Clinton to focus her campaign on slinging mud at Trump for his temperament.

Clinton, basking in the glow of her primary victories, could have taken the high road in her interview Wednesday with NBC anchor Lester Holt, rallying Democrats of all stripes by declaiming the progressive goals she would hope to achieve as president.

Instead, she dwelt in the interview, as she had last week in a speech ostensibly about foreign policy, on the flaws of her likely Republican rival.

On that same day after the final major primaries, the 2016 movie “13 Hours: The Secret Soldiers of Benghazi” was released to DVD and Amazon streaming, making the dramatization of the 2012 attack on the U.S. compound in the Libyan city available to a wide audience.

While the film, based on the 2014 book by Mitchell Zuckoff, a former award-winning journalist for the Boston Globe, does not delve into the political controversy around the incident, it shows clearly that the State Department’s initial explanation of a spontaneous demonstration was way wide of the mark.

All this now and we still have five months to go before Election Day.

Trump said Tuesday he would be making a speech next week with more details about how the Clintons have enriched themselves on the back of their public service.

It seems he will have plenty of material to draw on in attacking the character and temperament of the presumptive Democratic nominee.

Trump spokeswoman Katrina Pierson indicated as much in commenting on the Byrne book.

“The issue of temperament is more a problem for Hillary Clinton,” she told CNN. “I don’t recall Mr. Trump ever screaming at the Secret Service, calling them pigs, throwing vases across the room and clawing the face of his spouse, which Hillary Clinton has been reported to do.”

Clinton, for her part, if she has no better answers for the various charges against her than the ones she has given, would do better project a positive image of what her candidacy means to American voters.

Given what’s already happened this week, there seems to be little likelihood that she can win a mudslinging contest with Trump.

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